Don't become a stranger. Usually said between friends when parting in anticipation of not seeing one another for a long time - when moving house, for example.
Is also often as a joke after marriage - warning the couple not to lose touch with their friends after becoming so attached - normally between men.
By Rosamund
Don't Be A Stranger
Said to a person or workmate when they aren't going to be around anymore but not going to be far removed in a geographical sense. For example, if they are changing jobs but remaining in the same town or city.